1. Field
One embodiment of the present invention relates to a disk drive using a disk of discrete-track media type. More particularly, the invention relates to a technique of reproducing servo data from the disk.
2. Description of the Related Art
In the field of disk drives, a representative example of which is a hard disk drive, a disk of the discrete-track media type (hereinafter, referred to as a DTM disk or disk, as needed) has been attracting attention in recent years. (See, for example, Jpn. Pat. Apply. KOKAI Publication No. 2006-85819.) On the DTM disk, servo regions are provided, in which servo data is recorded. Each servo region has a projection-depression pattern consisting of projecting magnetic layers and concave nonmagnetic layers (actually, spaces).
The DTM disk has a data region that is the user-data recording region. The data region is composed of a number of data tracks that are annular magnetic layers. The data tracks are spaced apart from one another by spaces (nonmagnetic layers), which are called guard bands.
In the process of manufacturing the DTM disk, the surface of the unfinished disk is made flat and smooth after the projection-depression pattern consisting of projecting magnetic layers and depressions (spaces), in order to ensure the floating stability of the slider, i.e., the head body of the magnetic head. In the surface-smoothing process, nonmagnetic or soft-magnetic material is first filled in the grooves made in the surface of the DTM disk and then the unnecessary part of the material is removed by etching, imparting a flat surface to the DTM disk.
Here arises a problem. If a projecting magnetic layer (i.e., projecting region) formed in any the servo region has a width that is relatively broad (or long) or has a larger width than that of the data tracks (as measured in the radial direction of the DTM disk), the unnecessary part of the magnetic material cannot be thoroughly removed by the etching. Inevitably, the material remains on the projecting region. In many cases, the DTM disk cannot have a flat surface.
The above-identified publication proposes a method of imparting a sufficiently flat surface to the DTM disk. In this method, a groove (nonmagnetic layer) of a preset width is cut in each broad projecting region, thus splitting the projecting region into two regions. This method can indeed impart a flat surface to the DTM disk, but results in the following problem in the process of reproducing the servo data from the DTM disk.
In each broad projecting region on the DTM disk, a bit pattern, which is a part of the servo data, is formed. In the disk drive, the head reads or reproduces a signal from the projecting region, and the signal reproduced is processed, decoding the bit pattern. While the signal is processed, noise develops at the groove (nonmagnetic layer) cut in the broad projecting region. The noise may degrade the decoding efficiency in the reproduced-signal processing.